<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>

        function test(a, b) {
            console.log(a); // function a()
            console.log(b); // undefined
            var b = 234;
            console.log(b);  // 234
            a = 123;
            console.log(a);  // 123
            function a() { } 
            var a;
            b = 234;
            var b = function () { }
            console.log(a); // 123
            console.log(b); // function b()
        }

        test(1);
    </script>
</body>

</html>